1. Field of the Invention
This invention relates to apparatus responsive to arcing faults and circuit breakers incorporating same, and more particularly such apparatus preferably utilizing analog circuits.
2. Background Information
Conventional circuit breakers respond to overcurrents and short circuits to interrupt current in a protected circuit. Some circuit breakers also protect personnel and equipment from ground currents. Recently, there has been increased interest in providing protection against arcing faults. Arcing faults can occur for instance between adjacent bared conductors, between exposed ends of broken conductors, at a faulty connection and in other situations where conducting elements are in close proximity.
Arcing faults in ac systems can be intermittent as the magnetic repulsion forces generated by the arc current force the conductors apart to extinguish the arc. Mechanical forces then bring the conductors together again so that another arc is struck.
Arcing faults typically have high resistance so that the arcing current is below the instantaneous or magnetic trip thresholds of protection in a typical circuit breaker. Also, the intermittent nature of an arcing fault can create an average RMS current value which is below the thermal threshold for the circuit breaker Even so, the arcs can cause injury or start a fire if they occur near combustible material. It is not practical to simply lower the pick-up currents on conventional circuit breakers as there are many typical loads which draw similar currents, and would therefore, cause nuisance trips.
Much attention has been directed toward trying to distinguish arcing currents from other intermittent currents. It has been recognized that arcing currents generate a step increase in current when the arc is struck. However, many typical loads generate a similar step increase when a device is turned on. In many instances, the step increases generated by these loads are singular events while an arcing fault generates a series of step increases. One fault detector counts the step increases and generates a trip signal if a selected number of step increases occur within a given interval. However, there are loads such as a solid state dimmer switch which also generates step increases in current when the firing angle is phased back substantially.
It has also been recognized that arcing faults generate a great deal of high frequency noise, and further, that there are periods of quiescence in the high frequency component. Some circuit breakers look to such features to differentiate arcing faults from other phenomena. Circuit breakers which rely upon such detailed characteristics of current waveforms to detect arcing faults typically utilize a microprocessor to perform the analysis. They also require fairly good quality analog-to-digital converters to capture the high frequencies of interest. Thus, such arcing fault detectors add significantly to the cost of a circuit breaker, and in the case of a typical residential circuit breaker, can multiply its cost many times. Yet even such sophisticated arc detecting circuit breakers are subject to nuisance trips when confronted with some common load devices.
There is a need therefore, for an improved circuit breaker for providing protection against arcing faults which is economically practical. In this regard, there is a need for such a circuit breaker which can reliably respond to arcing faults and which does not require a microprocessor and associated high quality analog to digital converters.